Silicon etch rate enhancement by traces of metal

We report the effect of nickel and tungsten contamination on the etch behavior of silicon. This is studied in a molecular beam setup, where silicon is etched by $XeF2$ and $Ar+$ ions. The etch process is directly monitored by the $SiF4$ reaction products which leave the surface. The effect of contamination appears very pronounced after the ion beam is switched off: it leads to a temporary enhancement of the spontaneous etch rate on a time scale of 500 s. With traces of contamination on the order of 0.01 ML, the etch rate may be enhanced by a factor of 2 for W and somewhat less for Ni. It is concluded that the contamination moves into the silicon by diffusion to vacancies created by the $Ar+$ ions. For 1 keV $Ar+$ ions the contamination moves to a depth of 25 Å, comparable to the penetration depth of the ions. After etching a 170 Å thick layer, the catalytic effect of contamination is reduced to less than 5%. A simple model, which describes the measured effect of contamination very well, indicates that only 3% of the contamination is removed when a monolayer of silicon is etched away. Besides this catalytic effect there are indications that contamination can also lower the etch rate under certain conditions, because of the formation of silicides. From the measurements no conclusions could be drawn about the underlying mechanism of etch rate enhancement.